[plt-scheme] Stack Object

Hi all,

What's the difference between a method declared as interface and a 
public method of a given class? As example I'm checking stack class of 
the manual. There are three methods that are declared for the interface: 
push! pop! none?
I though this would be to provide some sort of abstraction layer and 
from here on we would only be able to call these methods but we can also 
call print-name that it is not in the interface. What's the utility of 
an interface?
An example would be great.

Best regards,

-- 
Paulo J. Matos : pocm(_at_)mega.ist.utl.pt
Instituto Superior Tecnico - Lisbon
Software & Computer Engineering - A.I.
  - > http://mega.ist.utl.pt/~pocm
  ---	
	Yes, God had a deadline...
		So, He wrote it all in Lisp!